Elemental analyses of feldspathic to basaltic soils and rocks on the moon using laser-induced breakdown spectroscopy

Abstract

In-situ analysis of major elements using laser-induced breakdown spectroscopy (LIBS) is essential for future lunar landing missions, yet its performance under lunar conditions remains not fully understood. This uncertainty arises from the absence of an atmosphere and the diverse range of surface materials, which vary in chemical composition from anorthosites to basalts, and in physical properties from fine regolith to boulders. To address these challenges, we developed and cross-validated a multivariate LIBS calibration model by measuring 169 compressed fine powders of geologic samples under vacuum. These samples fully encompass the bulk composition range of lunar meteorites. We investigated the applicability of the model to a wider range of samples by measuring lunar meteorites, terrestrial anorthites, and lunar simulants in various physical forms, including rock chips and soils with different grain sizes and bulk densities. For powder samples, the quantification accuracy, assessed using root mean squared error (RMSE), resulted in 2.5 wt% SiO₂, 0.25 wt% TiO₂, 1.2 wt% Al₂O₃, 1.3 wt% MgO, 1.2 wt% CaO, 0.33 wt% Na₂O, 0.47 wt% K₂O (0.060 wt% K₂O in the <1 wt% range), and 1.5 wt% T-Fe₂O₃. For rock chip samples, the RMSEs were 3.1 wt% SiO₂, 0.32 wt% TiO₂, 2.2 wt% Al₂O₃, 2.5 wt% MgO, 2.0 wt% CaO, 0.33 wt% Na₂O, 0.089 wt% K₂O, and 2.1 wt% T-Fe₂O₃. Despite significant differences in physical conditions between powders and rocks, their RMSEs remained consistent within a factor of two. Changes in grain size or bulk density of soils had relatively minor effects on the RMSE. These RMSEs confirm that the quantification accuracy of LIBS is sufficient to distinguish the subgroups within the lunar anorthosite suite (e.g., anorthosites vs. norites) and basalts (e.g., high-Ti vs. low-Ti) across a range of soil types, from coarse to fine and from loose to compact, as well as rocks. Furthermore, our analysis shows that LIBS can differentiate between “purest” and “pure” anorthosites (98 and 95 vol% plagioclase, respectively) based on the 3σ detection limits of Mg and Fe lines. These capabilities of LIBS align well with the goals of future lunar exploration, such as locating ilmenite-rich soils for resource extraction, detecting purest anorthosites to understand early lunar evolution, and identifying noritic impact melts to refine lunar chronology. Overall, our results demonstrate that LIBS serves as a versatile tool for rapid geochemical characterization on the Moon.